U.S. patent number 7,046,423 [Application Number 10/345,434] was granted by the patent office on 2006-05-16 for separation of encapsulated particles from empty shells.
This patent grant is currently assigned to Xerox Corporation. Invention is credited to Naveen Chopra, Jurgen Daniel, Peter M. Kazmaier, Armin R. Volkel.
United States Patent |
7,046,423 |
Volkel , et al. |
May 16, 2006 |
Separation of encapsulated particles from empty shells
Abstract
Methods and systems for separating encapsulated particles from
empty shells. One method involves providing a mixture including at
least one dipolar particle encapsulated in a shell and at least one
shell which does not encapsulate a dipolar particle. The mixture is
positioned in a spatially inhomogeneous electric or magnetic field
and at least one encapsulated dipolar particle is isolated from the
mixture.
Inventors: |
Volkel; Armin R. (Mountain
View, CA), Kazmaier; Peter M. (Mississauga, CA),
Chopra; Naveen (Oakville, CA), Daniel; Jurgen
(Mountain View, CA) |
Assignee: |
Xerox Corporation (Stamford,
CT)
|
Family
ID: |
32711920 |
Appl.
No.: |
10/345,434 |
Filed: |
January 15, 2003 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20040135743 A1 |
Jul 15, 2004 |
|
Current U.S.
Class: |
359/296; 264/4.3;
345/107; 428/402.21 |
Current CPC
Class: |
G02B
26/026 (20130101); Y10T 428/2985 (20150115) |
Current International
Class: |
G02B
26/00 (20060101); B01J 13/02 (20060101); B32B
9/00 (20060101); G09G 3/34 (20060101) |
Field of
Search: |
;264/4,4.1,4.3 ;345/107
;359/296 ;428/402.2,402.21,402.24 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
E INK Web Pages "Technology" and "Active Matrix Displays" (printed
Dec. 16, 2002). cited by other .
"E INK, Toppan and Philips Demonstrate World's First High
Resolution, Active-Matix Color, Display with Electronic Ink," Press
Release (Jul. 1, 2002). cited by other.
|
Primary Examiner: Spector; David N.
Attorney, Agent or Firm: Nixon Peabody LLP
Claims
What is claimed is:
1. A method of separating encapsulated dipolar particles from empty
shells comprising: providing a mixture comprising at least one
dipolar particle encapsulated in a shell and at least one shell
which does not encapsulate a dipolar particle; positioning the
mixture in a spatially inhomogeneous electric or magnetic field;
and isolating at least one encapsulated dipolar particle from the
mixture.
2. The method according to claim 1 wherein at least one dipolar
particle is a bichromal ball.
3. The method according to claim 1 wherein the shell is a polymer
shell.
4. The method according to claim 1 wherein the mixture comprises a
plurality of encapsulated dipolar particles.
5. The method according to claim 1 wherein the mixture comprises a
plurality of shells which do not encapsulate a dipolar
particle.
6. The method according to claim 1 wherein at least one
encapsulated dipolar particle and at least one shell which does not
encapsulate a dipolar particle have a monopolar electric charge and
the spatially inhomogeneous electric field is an alternating
current electric field.
7. The method according to claim 1 wherein positioning comprises
passing the mixture through a spatially inhomogeneous electric or
magnetic field directed in a non-parallel direction to a direction
of motion of the mixture.
8. The method according to claim 7 wherein passing comprises
passing through the spatially inhomogeneous electric or magnetic
field in a fluid carrier.
9. The method according to claim 7 wherein passing comprises
gravitational passage through the spatially inhomogeneous electric
or magnetic field.
10. The method according to claim 1 wherein isolating comprises
collecting at least one encapsulated dipolar particle and at least
one shell which does not encapsulate a dipolar particle in separate
reservoirs positioned at an end of the spatially inhomogeneous
electric or magnetic field.
11. A system for separating encapsulated dipolar particles from
empty shells comprising: a mixture comprising at least one dipolar
particle encapsulated in a shell and at least one shell which does
not encapsulate a dipolar particle, and an apparatus which provides
a spatially inhomogeneous electric or magnetic field, wherein the
spatially inhomogeneous electric or magnetic field is directed in a
non-parallel direction to a direction of motion of the mixture to
isolate at least one encapsulated dipolar particle.
12. The system according to claim 11 wherein at least one dipolar
particle is a bichromal ball.
13. The system according to claim 11 wherein the mixture comprises
a fluid carrier.
14. A method of separating encapsulated particles from empty shells
comprising: providing a mixture comprising at least one particle
having an electric charge encapsulated in an electrically neutral
shell and at least one electrically neutral shell which does not
encapsulate a particle; positioning the mixture in an electric
field; and isolating at least one encapsulated particle from the
mixture.
15. The method according to claim 14 wherein the shell is a polymer
shell.
16. The method according to claim 14 wherein the mixture comprises
a plurality of particles having an electric charge each
encapsulated in an electrically neutral shell.
17. The method according to claim 14 wherein the mixture comprises
a plurality of electrically neutral shells which do not encapsulate
a particle.
18. The method according to claim 14 wherein at least one
encapsulated particle has an induced dipole moment different from
at least one electrically neutral shell which does not encapsulate
a particle in the electric field.
19. The method according to claim 14 wherein positioning comprises
passing the mixture through an electric field directed in a
non-parallel direction to a direction of motion of the mixture.
20. The method according to claim 19 wherein passing comprises
passing through the electric field in a fluid carrier.
21. The method according to claim 19 wherein passing comprises
gravitational passage through the electric field.
22. The method according to claim 14 wherein isolating comprises
collecting at least one encapsulated particle and at least one
electrically neutral shell which does not encapsulate a particle in
separate reservoirs positioned at an end of the electric field.
23. A system for separating encapsulated particles from empty
shells comprising: a mixture comprising at least one particle
having an electric charge encapsulated in an electrically neutral
shell and at least one electrically neutral shell which does not
encapsulate a particle, and an apparatus which provides an electric
field, wherein the electric field is directed in a non-parallel
direction to a direction of motion of the mixture to isolate at
least one encapsulated particle.
24. The system according to claim 23 wherein the mixture comprises
a fluid carrier.
Description
FIELD OF THE INVENTION
The present invention relates to the separation of particles
encapsulated in a shell from unfilled shells.
BACKGROUND OF THE INVENTION
Bichromal balls have two hemispheres, typically one black and one
white, each having different electrical properties. Such bichromal
balls are frequently used in a "twisting ball" medium for
displaying an image. The twisting ball medium includes internal
bichromal balls that rotate to show either black or white
hemispheres in response to an externally applied electrical field
which are contained in individual liquid filled spherical cavities
in a transparent binder, such as a gel. The gel is then bonded
between glass or plastic sheets for protection.
Alternatively, such bichromal balls may be enclosed within
individual spherical shells and then a space between the ball and
shell is filled with a liquid to form a microsphere so that the
ball is free to rotate in response to an electrical field. The
microspheres can then be mixed into a substrate which can be formed
into sheets or can be applied to any kind of surface. The result is
a film which can form an image from an applied electrical field.
The display formed using this technique allows the formation of a
thin, paper-like sheet without the bulkiness and optical problems
created by the need for protective cover sheets in a twisting ball
medium. In digital document media, the bichromal balls are embedded
in a gel matrix. By applying an external electric field, the
bichromal balls are rotated to direct either of their two
differently colored sides to an observer.
However, with the current processes for encapsulation, a number of
empty shells (i.e., shells which do not encapsulate a bichromal
ball) are generated with the individual encapsulated bichromal
balls. Because both the empty and filled shells have about the same
mass, they can not easily be separated by typical sedimentation
processes. However, since only filled shells produce an image, the
inclusion of empty shells in a medium reduces the image quality.
Therefore, for applications requiring high image quality, empty
shells will need to be separated from a mixture containing empty
shells and filled shells.
SUMMARY OF THE INVENTION
The present invention relates to a method of separating
encapsulated dipolar particles from empty shells. This method
involves providing a mixture including at least one dipolar
particle encapsulated in a shell and at least one shell which does
not encapsulate a dipolar particle. The mixture is positioned in a
spatially inhomogeneous electric or magnetic field and at least one
encapsulated dipolar particle is isolated from the mixture. The
dipolar particles include particles having a permanent dipole
moment and particles having an induced dipole moment. The dipolar
particles also include particles having an electric dipole moment
and particles having a magnetic dipole moment.
Another aspect of the present invention relates to a system for
separating encapsulated dipolar particles from empty shells. The
system includes a mixture including at least one dipolar particle
encapsulated in a shell and at least one shell which does not
encapsulate a dipolar particle and an apparatus which provides a
spatially inhomogeneous electric or magnetic field, wherein the
spatially inhomogeneous electric or magnetic field is directed in a
non-parallel direction to a direction of motion of the mixture to
isolate at least one encapsulated dipolar particle from the
mixture.
The present invention also relates to a method of separating
encapsulated particles from empty shells. This method involves
providing a mixture comprising at least one particle having an
electric charge encapsulated in an electrically neutral shell and
at least one electrically neutral shell which does not encapsulate
a particle. The mixture is positioned in an electric field and at
least one encapsulated particle is isolated from the mixture.
Yet another aspect of the present invention relates to a system for
separating encapsulated particles from empty shells. The system
includes a mixture including at least one particle having an
electric charge encapsulated in an electrically neutral shell and
at least one electrically neutral shell which does not encapsulate
a particle and an apparatus which provides an electric field,
wherein the electric field is directed in a non-parallel direction
to a direction of motion of the mixture to isolate at least one
encapsulated particle from the mixture.
The methods and systems of the present invention allow for the
separation of encapsulated particles, e.g., bichromal balls, from
unfilled shells. In particular, with the methods of the present
invention, at least 20% of the unfilled shells may be separated
from a mixture of encapsulated bichromal balls and unfilled shells.
This allows for the production of high image quality displays.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of the flow directions and force
directions for the separation of encapsulated bichromal balls by
fluid flow;
FIGS. 2A B are schematic diagrams of a system for separation of
encapsulated bichromal balls by fluid flow in accordance with one
embodiment of the present invention;
FIG. 3 is a schematic diagram of the flow directions and force
directions for the separation of encapsulated bichromal balls by
gravitational passage;
FIG. 4 is a schematic diagram of a system for separation of
encapsulated bichromal balls by sedimentation in accordance with
one embodiment of the present invention; and
FIGS. 5A B are schematic diagrams of electrode configurations for
generating a spatially inhomogeneous electric field.
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to a method of separating
encapsulated dipolar particles from empty shells. This method
involves providing a mixture including at least one dipolar
particle encapsulated in a shell and at least one shell which does
not encapsulate a dipolar particle. The mixture is positioned in a
spatially inhomogeneous electric or magnetic field and at least one
encapsulated dipolar particle is isolated from the mixture. As used
herein, a mixture describes a combination of two or more components
in varying proportions that retain their own properties.
Another aspect of the present invention relates to a system for
separating encapsulated dipolar particles from empty shells. The
system includes a mixture including at least one dipolar particle
encapsulated in a shell and at least one shell which does not
encapsulate a dipolar particle and an apparatus which provides a
spatially inhomogeneous electric or magnetic field, wherein the
spatially inhomogeneous electric or magnetic field is directed in a
non-parallel direction to a direction of motion of the mixture to
isolate at least one encapsulated dipolar particle from the
mixture.
The present invention also relates to a method of separating
encapsulated particles from empty shells. This method involves
providing a mixture comprising at least one particle having an
electric charge encapsulated in an electrically neutral shell and
at least one electrically neutral shell which does not encapsulate
a particle. The mixture is positioned in an electric field and at
least one encapsulated particle is isolated from the mixture.
Yet another aspect of the present invention relates to a system for
separating encapsulated particles from empty shells. The system
includes a mixture including at least one particle having an
electric charge encapsulated in an electrically neutral shell and
at least one electrically neutral shell which does not encapsulate
a particle and an apparatus which provides an electric field,
wherein the electric field is directed in a non-parallel direction
to a direction of motion of the mixture to isolate at least one
encapsulated particle from the mixture.
Referring to FIG. 1, a method and system in accordance with one
embodiment of the present invention is shown. In particular,
separation of a plurality of encapsulated bichromal balls in
laminar fluid flow is shown in FIG. 1. As shown in FIG. 1, all
capsules (i.e., a plurality of filled shells and a plurality of
empty shells) are provided in a fluid carrier. As used herein, the
empty shells do not encapsulate a bichromal ball, but may
encapsulate other materials (e.g., the "empty shells" typically
encapsulate liquid used to produce a microsphere). Suitable fluid
carriers include, but are not limited to, water, Isopar oil,
silicon oil, isopropyl alcohol, hexanes, toluene, and mixtures
thereof. In one embodiment, the fluid carrier is a non-conductive
fluid having a low dielectric constant. The fluid carrier may be
either liquid or gas.
The mixture of capsules in the fluid carrier is transported through
a spatially inhomogeneous electric field (SIEF). As shown in FIG.
1, the fluid, and with it all of the capsules, moves with the speed
u.sub.f in the x direction. The electric field gradient
(.differential..sub.yE) is parallel to the y direction. Thus, in
this embodiment, the electric field is directed substantially
perpendicular to the direction of motion of the fluid and capsules.
However, the electric field may be directed in any non-parallel
direction to the direction of motion of the fluid and capsules,
although the separation distance .DELTA.y between filled and empty
capsules increases faster for a given time interval .DELTA.t, when
the electric field is applied more perpendicular to the direction
of motion. Therefore, the most efficient way to use the SIEF is in
a direction substantially perpendicular to the direction of motion.
The encapsulated bichromal balls have a permanent electric dipole
moment, while the empty shells do not. Thus, the encapsulated
bichromal balls experience an additional force from the SIEF and
their trajectory bends in the direction of the electric field
gradient. The empty shells do not experience an additional force
from the SIEF and, therefore, travel throughout the electric field
gradient substantially parallel to the flow field (i.e., in the x
direction). The change in trajectory of the encapsulated bichromal
balls can then be used to separate or isolate the encapsulated
bichromal balls from the empty shells.
The total deflection .DELTA.y of the encapsulated bichromal balls
from a straight trajectory depends upon their dipole moment p, the
strength of the electric field gradient (.differential..sub.yE),
and the time interval .DELTA.t the capsules travel through the
electric field. For a constant electric field gradient, the
deflection is estimated as
.DELTA..times..times..alpha..times..DELTA..times..times..times..times..di-
fferential..times..alpha..times..times..pi..times..times..eta..times..time-
s. ##EQU00001## .eta. is the dynamic viscosity of the fluid, and r
and m are the particle size and mass, respectively. Because of the
similar density of the bichromal balls and the encapsulated liquid
in the "empty shells," the mass difference of the filled and empty
shells is assumed negligent.
After passage through the SIEF, the isolated, encapsulated
bichromal balls may be separated from the fluid carrier. Techniques
for separating the encapsulated balls from the fluid carrier are
known in the art and include, for example, filtration. Suitable
filters include, but are not limited to fibrous filters and metal
screens. The filters may include a pore size of approximately 20
.mu.m, however, any suitable pore size which allows passage of the
fluid but not the encapsulated balls through the filter may be
used.
A system for separation by laminar fluid flow in accordance with
one embodiment of the present invention is shown schematically in
FIGS. 2A B. In particular, the system includes a first electrode 10
(e.g., cathode) and a second electrode 12 (e.g., anode) connected
to a power supply 14 to generate an electric field. Examples of
electrode configurations are described later with reference to
FIGS. 5A B. A supply tank 16 of un-separated capsules is connected
to a supply conduit of un-separated capsules 18. A supply tank of
fluid carrier 20 is connected to a supply conduit for fluid carrier
22 through a valve 24. The supply conduits 18, 22 allow the mixture
of un-separated capsules and fluid carrier to pass through the
electric field generated between electrodes 10 and 12. The empty
capsules follow trajectory 26 and pass into exit conduit 28, while
the filled capsules follow trajectory 30 and pass into exit conduit
32. The empty capsules are collected in reservoir 34 and the filled
capsules are collected in reservoir 36. The coordinate axis as
shown in FIG. 1 are labeled as 38 in FIG. 2A for reference. After
separation, the fluid carrier may be recycled back into the supply
tank of fluid carrier, as shown by arrows 40 in FIG. 2B.
A second embodiment of a method and system of the present invention
is shown in FIG. 3. In this particular embodiment, a mixture
comprising a plurality of encapsulated bichromal balls and at least
one shell which does not encapsulate a bichromal ball is separated
using gravitational force through air or a vacuum. In particular,
as shown in FIG. 3, the mixture is dropped in a container (e.g., a
tube) and the capsules (i.e., encapsulated bichromal balls and
empty shells) fall due to gravity. More specifically, the capsules
fall in the -z direction due to the gravitational acceleration g.
The SIEF gradient (.differential..sub.yE) is directed substantially
perpendicular to the direction of motion of the mixture (i.e., the
direction of the fall) and parallel to the y direction. Again, as
described above, the encapsulated bichromal balls will experience
the additional force from the SIEF and their trajectory will bend
in the direction of the electric field gradient. More specifically,
after falling a distance h through the electric field, the
encapsulated balls deflect by a distance .DELTA.y from the straight
path of the empty shell(s).
When this separation is performed in air, the falling capsules will
reach a terminal velocity (g/.alpha.) determined by the balance of
the gravitational and drag forces on the capsules. When the
encapsulated bichromal balls travel a distance h through a constant
electric field gradient, their total deflection is estimated as
.DELTA..times..times. ##EQU00002## The particle deflection shown in
the above equation has been calculated assuming the particles have
reached terminal velocity before entering the electric field. When
they continue to accelerate while moving through the electric
field, the deflection is larger due to the longer time they are
exposed to the electric field gradient. When the separation is
performed in a vacuum, the particle deflection is exactly as given
by the above equation (for a constant electric field gradient).
Separation in a vacuum eliminates the possibility of turbulent flow
interfering with the separation.
A third embodiment of a method and system of the present invention
relates to separation through a liquid due to gravitational force
(i.e., sedimentation). A schematic showing one embodiment of a
system for implementation of this method is shown in FIG. 4. The
system includes a separation tank 50 filled with liquid 52. A first
electrode 54 (e.g., cathode) and a second electrode 56 (e.g.,
anode) are connected to a power supply 58 to generate an electric
field. A supply tank of un-separated capsules 60 is connected to
the separation tank 50. The un-separated capsules pass through the
electric field generated by electrodes 54 and 56. The filled
capsules follow trajectory 62 into reservoir 64, while the empty
capsules follow trajectory 66 into reservoir 68. The reservoirs 64
and 68 include valves 70 and 72, respectively. In this particular
embodiment, the system includes a reservoir 74 which holds
additional liquid 52 and a control system 76 to maintain the liquid
level in separation tank 10 at a constant level.
This embodiment of the present invention is similar to that
described with reference to FIG. 3. The coordinate axis of FIG. 3
are also shown in FIG. 4 as 78 for reference. However, with regard
to the embodiment described with reference to FIG. 3, the liquid 52
would be replaced with air or a vacuum. When a vacuum is utilized,
the tank 50 is sealed and the liquid reservoir 74 and control
system 76 is replaced with a vacuum pump.
In the embodiment shown in FIG. 4, the capsules include
encapsulated bichromal balls and empty shells which are dropped in
a liquid that has a density .rho..sub.l that is less than the
density of the capsules .rho..sub.c. (When the density of the
liquid is larger than the density of the capsules, the capsules
will rise with constant speed. This could also be used for
separation). Due to the gravitational force, the particles will
descend with a constant velocity given by the balance of
gravitational and friction force
.times..times..function..rho..rho..times..times..eta. ##EQU00003##
where g=9.81 m/s.sup.2 is the gravitational acceleration, .eta. is
the viscosity of the liquid, and r is the radius of one
capsule.
In this particular embodiment, the SIEF is applied substantially
perpendicular to the direction of sedimentation. The filled
capsules will deflect due to their dipole moment. For a
sedimentation distance h, the filled capsules will experience a
lateral deflection of
.DELTA..times..times..times..rho..rho..rho..times..differential..times.
##EQU00004## p is the dipole moment, .differential..sub.yE is the
gradient of the electric field perpendicular to the direction of
sedimentation, and m is the mass of one capsule.
Though this method and system is similar to that described with
reference to FIG. 3, sedimentation in a liquid has the advantage
that the velocity with which the capsules move through the SIEF can
be controlled by choosing a liquid with the appropriate density. In
particular, the sedimentation speed depends on the difference in
density between the capsules and the liquid. This allows the time
that the capsules spend inside the SIEF to be maximized, hence
allowing for sufficient separation before extracting the capsules
from the liquid. More specifically, the liquid can be chosen to
allow the capsules to move very slowly through the electrical
field, thus substantial separation can be achieved with shorter
(potentially less expensive) electrodes. Thus, suitable liquids in
accordance with this embodiment of the present invention are
determined by the capsules being separated. Examples of suitable
liquids include, but are not limited to, water, Isopar oil, silicon
oil, isopropyl alcohol, hexanes, toluene, and mixtures thereof.
For a situation where the capsules (both filled and empty) have an
additional monopole moment, all particles will experience a
deflection inside the electric field. A possible method to
compensate for this is aligning the electrodes that provide the
electric field gradient at an angle .phi. to the direction of the
driving force (i.e., the direction of the fluid flow or
gravitational force) (see 80 in FIG. 4). For the sedimentation
case, the angle is defined by
.times..times..times..PHI..alpha..+-..alpha..times..alpha.
##EQU00005## with .alpha.=4.pi.r.sup.3
(.rho..sub.c-.rho..sub.l)g/3QE, and Q is the monopole charge on the
capsules. In this case, the balance of the gravitational and
electrical forces make the capsules move on a straight line that is
substantially perpendicular to the SIEF field. However, the
additional dipolar moment of the filled capsules will lead to a
deviation from this straight line that will allow separation of
those capsules from the unfilled capsules. Another method to
compensate for the monopole moments on the capsules is to apply an
alternating current (ac) voltage to the electrodes. The monopole
charge of the capsules will make the particles change direction
with the electric field, hence leading to time average zero
displacement. However, particles with a dipole moment will always
experience an additional force into the direction of increasing
field gradient, hence allowing separation as described above. This
is also true if each capsule has a different monopole charge. The
only requirement for the ac voltage is that its frequency be low
enough so that the dipoles inside the capsule have time to realign
themselves with the new field direction. Possible sources of
monopole charge on the capsules include tribo-charging of dry
shells, or dissociation of surface molecule in a liquid
environment. In the second case, charge control agents can be added
to the solution to adjust/minimize any monopole charge.
Isolating at least one encapsulated particle after any of the above
separations is achieved by any suitable method, such as collecting
the empty and filled capsules in separate reservoirs positioned at
the end of the electric field. Once isolated, the encapsulated
particles may be used as desired, e.g., incorporated in display
media.
Although the invention has been described with reference to
bichromal balls, the methods and systems of the present invention
may be used with any particles having an electric charge. In
particular, the particles for use in the methods and systems of the
present invention may be monopolar, dipolar, or both. Capsules that
have a monopolar charge that is different between filled and empty
capsules may be separated by both spatially homogeneous and
spatially inhomogeneous fields (and constant and non-constant
electric fields). Neutral shells that are filled with a particle
with a permanent dipole moment may be separated from empty capsules
by a spatially inhomogeneous field, as described above. Techniques
and apparatuses for generating suitable electric fields for
separation of monopolar particles are known in the art and will not
be described in detail herein (see, e.g., Jackson, Classical
Electrodynamics, John Wiley & Sons (1998), which is hereby
incorporated by reference in its entirety).
Moreover, dipolar particles in accordance with the present
invention may have a permanent dipole moment (e.g., bichromal
balls) or an induced dipole moment. For particles having an induced
dipole moment, the dipole moment is dependent upon the electric
field used. In particular, any dielectric particle will develop an
induced dipole moment when exposed to an electric field E that is
proportional to the field: p.sub.ind=.gamma.E. .gamma. is the
polarizability and describes how easily a material is polarized in
an applied field. Systems with mobile charges, e.g., encapsulated
electrophoretic inks that contain nano- or micron-sized particles
of opposite charge, provide a large induced dipole moment at low
applied fields. Neutral capsules that have a difference in
polarizability between filled and empty capsules may be separated
by a spatially inhomogeneous field, since the induced dipole moment
due to the applied field will be different in this case. This
method works for both electric and magnetic dipole moments
(permanent or induced) in an electric or magnetic field,
respectively.
Dipolar particles in accordance with the present invention may have
an electric dipole moment (e.g., bichromal balls) or a magnetic
dipole moment. Examples of suitable dipolar particles having a
magnetic dipole moment include, but are not limited to, magnetite
nano- or micro-particles, nano- or micron-sized particles of cobalt
in an organic carrier, and the like (see, e.g., Rosenweig,
Ferrohydrodynamics, Dover Publications, Inc. (1997), which is
hereby incorporated by reference in its entirety). In the case of
particles having a magnetic dipole moment, a spatially
inhomogeneous magnetic field would be used for separation.
Techniques and apparatuses for producing such magnetic fields are
known in the art and will not be described detail herein (see,
e.g., Jackson, Classical Electrodynamics, John Wiley & Sons
(1998), which is hereby incorporated by reference in its
entirety).
In addition, any suitable shell may be used in the methods of the
present invention. For example, polymer shells (e.g., transparent
polymer shells and opaque polymer shells), shellac, epoxy, and
glass may be used as shells in the present invention. Examples of
suitable polymers for the polymer shells include, but are not
limited to, polyurethanes, polystyrenes, polymethylmethacrylate,
gelatin-gum Arabic, gelatin-polyphosphate, and polyureas. Other
examples of shell materials are described in U.S. Pat. No.
6,067,185, which is hereby incorporated by reference in its
entirety.
With regard to the embodiment of the present invention relating to
separation of bichromal balls, the mixture of at least one
encapsulated bichromal ball and at least one empty shell may be
provided by methods known in the art. For example, methods for
producing bichromal balls are described in commonly assigned U.S.
Pat. Nos. 4,126,854, 4,143,103, 5,075,186, 5,262,098, 5,344,594,
5,389,945, 5,604,027, and 5,708,525, which are hereby incorporated
by reference in their entirety.
Further, although the above-described methods and systems of the
present invention relate to the passage of encapsulated particles
and empty shells through electric or magnetic fields (e.g., by
laminar fluid flow or by gravitational passage), separation can be
achieved without passage of the particles through the field. In
particular, by simply positioning the mixture of filled and empty
capsules in an electric or magnetic field, the capsules will
separate and can then be isolated. However, passage through an
electric or magnetic field allows for a continuous method of
separation.
In the methods and systems described above, at least 20% of the
unfilled shells may be separated from a mixture of filled and
unfilled shells. In particular, in one embodiment, from about 20%
to about 80% of unfilled shells may be separated.
Suitable techniques and apparatuses for generating a SIEF in the
methods and systems of the present invention are shown in FIGS. 5A
B. The apparatuses shown in FIGS. 5A B may also include a suitable
housing (e.g., a tube), as is known in the art. In particular, FIG.
5A shows an electrode configuration matching the Stern Gerlach
experiment (separating free electrons by their magnetic dipole
moment) which produces a strong, approximately constant, magnetic
field gradient with the magnet geometry shown. The forces of an
electric field on an electric dipole and of a magnetic field on a
magnetic dipole are of the same form, namely p.differential..sub.yE
and m.differential..sub.yB, respectively, where m is the magnetic
dipole moment and .differential..sub.yB is the magnetic field
gradient, and the requirements on magnet geometry are the same for
the electrode configuration to achieve a constant field gradient.
As shown in FIG. 5A, the electrode configuration includes anode
(1), cathode (2), and electric field lines (3).
An alternative electrode configuration is shown in FIG. 5B. In this
figure, two concentric cylinders are shown. The outer cylinder (2)
is hollow and the inner cylinder (1) can be a wire. In inner
cylinder (1) is the anode, the outer cylinder (2) is the cathode,
and the electric field lines are shown as (3). The capsules move
parallel to the axis between the two cylinders. However, this
electrode configuration will produce an electric field gradient
that decreases with the inverse distance squared (i.e., a
non-constant field gradient). The SIEF is calculated as
follows:
.differential..times..ident..differential..differential..function..times.
##EQU00006##
Assuming over-damped dynamics (i.e., where the time scale or
velocity changes of the particles due to the electric field
gradient is much larger than the drag time scale 1/.alpha.) the
deflections are estimated as
.DELTA..times..times.
.times..alpha..times..times..DELTA..times..times. .times.
##EQU00007## for separation in laminar fluid flow and separation in
free fall through air or vacuum, respectively, and with
.times..function. ##EQU00008## Here, the total displacement is
increasing much less with L or h, p, and (.differential..sub.yE),
as compared to the case with the constant electric field gradient.
On the other hand, equations (10) and (11) are results for the
over-damped case and present a lower limit of the total deflection
of the encapsulated particles. When the radius of the inner
cylinder in the electrode configuration of FIG. 5B is very small,
the electric field gradient will become very strong and the capsule
dynamics will not be over-damped, leading to larger total
deflection.
EXAMPLES
Example 1
Typical parameters for the method of separation as shown in FIG. 5B
for bichromal balls are set forth in Table 1, below.
TABLE-US-00001 TABLE 1 Parameter Size Radius of Ball 30 .mu.m
Dipole Moment 1.5 * 10.sup.-17 Cm (50 fC separated by 30 .mu.m,
permanent or induced) Radius of Shell 50 .mu.m Density 10.sup.3
kg/m.sup.3 V.sub.a - V.sub.i 1000 V Inner Electrode Radius 0.5 mm
Outer Electrode Radius 10 mm
Using these parameters and the deflection formulas derived above,
Tables 2 and 3 set forth the (minimal) deflections for the
encapsulated balls.
TABLE-US-00002 TABLE 2 Separation In A Fluid With Viscosity of
10.sup.-3 Pas Travel Time Deflection Through SIEF (s) (mm) 1 0.8 10
1.7 100 3.8 1000 8
TABLE-US-00003 TABLE 3 Separation In Free Fall Through Air Or
Vacuum Travel Distance Deflection Through SIEF (m) (mm) 0.25 2.9
0.5 3.7 1.0 4.6
TABLE-US-00004 TABLE 4 Separation Through Sedimentation In Liquid
With .rho..sub.1 = 0.99 .rho..sub.c Travel Distance Deflection
Through SIEF (m) (mm) 0.01 4.6 0.05 7.9 0.1 9.9
Other modifications of the present invention may occur to those
skilled in the art subsequent to a review of the present
application, and these modifications, including equivalents
thereof, are intended to be included within the scope of the
present invention. Further, the recited order of processing
elements or sequences, or the use of numbers, letters, or other
designations therefor, is not intended to limit the claimed process
to any order except as may be specified in the claims.
* * * * *